Zirconium oxide is a useful raw material used for various applications. Specifically, zirconium oxide is used as a ceramic raw material in the fields of refractories, ceramic capacitors, oxygen sensors, piezoelectric materials, structures, solid oxide-type fuel cells, catalysts, coating compositions, binders, optical materials, coating agents, and the like.
When zirconium oxide is used as a raw material to obtain various products, the zirconium oxide is, in many cases, used after mixing and compounding with other materials. In these cases, the smaller the average particle diameter of the zirconium oxide and the more monodisperse the zirconium oxide is, the easier it is to mix and compound the zirconium oxide with other materials. When mixing and compounding are sufficient, the resulting composite has few local deviations in the composition. Thus, an improvement in product performance can be expected. That is, in order to pursue the improvement in product performance, it is necessary to sufficiently perform mixing and compounding to obtain more homogeneous composites.
Monodisperse zirconium oxide, which has a small average particle diameter, is desired in the field concerned; however, it is very difficult to control the aggregation of zirconium oxide. It is particularly difficult to obtain monodisperse zirconium oxide having an average particle diameter of several 100 nm or less.
To address the above problems, zirconium oxide sols can control aggregation by taking advantage of electrostatic repulsion between the sol particles; thus, monodisperse zirconium oxide sols having an average particle diameter of several 100 nm or less can be realized. These characteristics are unique to zirconium oxide sols, and zirconium oxide sols are therefore suitably used in the field concerned.
When the average particle diameter of the sol particles of the zirconium oxide sol is controlled to be small, it is advantageous in terms of surface activity due to a high specific surface area for applications concerning catalysts, structures, binders, etc. Specifically, sol particles with a smaller average particle diameter contribute to, for catalysts, an increase in the reaction rate, and for structures, a decrease in the generation temperature; for binders, a smaller amount of such sol particles contributes to high caking capacity etc.
The following zirconium oxide sols and methods for producing the same are disclosed.
PTL 1 discloses a colloidal sol in which most of the zirconium oxide particles are monoclinic crystals, the primary particle diameter is 3 to 10 nm, and the average diameter of secondary aggregate particles does not exceed 50 nm. PTL 1 also indicates that the colloid sol is obtained by adding hydrogen peroxide or a compound that produces hydrogen peroxide to a zirconium salt aqueous solution having a concentration of 0.05 to 2.0 mol/L, and heating the resulting solution to 80 to 300° C.
PTL 2 discloses a production method that provides nanoparticles of 10 nm or less by controlling the aggregation of single nanoparticles using a polymer.
PTL 3 discloses a method for producing an amorphous zirconium oxide sol having a particle diameter of 1 to 20 nm.
This method is realized by controlling the zirconium concentration and the amount of nitric acid.
PTL 4 proposes a method for obtaining a composite sol by hydrolyzing a mixed solution of zirconium, titanium, and tin; however, the dispersed state of each element is nowhere described, and the particle diameter is as wide as 1 to 100 nm. Moreover, this method incurs many problems; for example, the use of metal tin etc. may lead to the generation of hydrogen.
Thus, PTL 1 to PTL 4 disclose methods for producing zirconium oxide sols; however, these methods incur problems, such as the necessity of adding polymers in order to maintain single nanoparticles in a highly dispersed state, and the limited acid concentration of the zirconium salt solution. There has been no method that can easily realize single nanoparticles. Furthermore, no method has been found to essentially control the problem of crystal growth of zirconium oxide particles. Accordingly, it is not easy to mass-produce single nano-level zirconium oxide sols.